This invention is concerned with an injection liner, and relates in particular to a novel form of liner utilisable with a septum and cap and intended for use in the injector of a gas chromatograph.
Gas chromatography is a technique widely employed in industry. It finds particular application in the fields of medical care, pharmaceutical analysis, petroleum chemistry, petrochemicals and environmental analysis, and is especially valuable for the separation of complex mixtures into their components, typically such mixtures as contain organic chemicals, either as a simple mixture or in a solvent, such as water.
Basically, chromatography involves the transferring of a mixture of materials (the xe2x80x9csample mixturexe2x80x9d) by means of a flowing medium (the xe2x80x9cmobile phasexe2x80x9d) along a passageway (the xe2x80x9ccolumnxe2x80x9d) containing a substance (the xe2x80x9cstationary phasexe2x80x9d) to which the different components of the mixture are weakly, but differentially, attracted, so that gradually the less attracted components get ahead of the more attracted ones, and eventually, if the column is long enough, they are completely, separated. It is called xe2x80x9cchromatographyxe2x80x9dxe2x80x94colour writingxe2x80x94after its original use, which was to separate mixtures of coloured dyes. In the case of gas chromatography, the mobile phase is a gas.
There are a number of different types of gas chromatography, but those in common use employ as the column a long tube containing the stationary phase. There are several designs of column, but typically it is long, narrow tube made from fused silica (like glass) coated on the outside with a polyimide or similar plastic layer to prevent corrosion and add strength. The silica tube may typically be from 5 to 50 meters (about 16 to 160 ft) long, and be of internal diameter 100 to 750 microns (0.0001-0.00075 m, or 0.01-0.075 cm, or about 0.004-0.03 in). The stationary phase chemical is either bonded on the inside of this tube or, in some cases, is deposited on solid, porous inert support materials which fill the tube.
When a small amount of the sample mixture is introduced to the inlet end of the column, with the mobile phase passing from the inlet end of the column to the outlet end, the mixture is blown slowly through the column. The stationary phase selectively slows down some of the compounds in the sample mixture, whereas other compounds are slowed much less or not at all. Compounds that are not slowed travel at the speed of the mobile phase, while compounds that are slowed down travel more slowly than the mobile phase. Because some compounds are slowed down and others are slowed less, those which are slowed less emerge from the far end of the column before those which are slowed more. Thus the mixture becomes separated into (some of) its components
In operation, a very small amount of the sample mixturexe2x80x94typically one milligram or lessxe2x80x94is injected into the inlet end of the column which is sealed into ant elongate tubular device called an injector. The injector facilitates the introduction of this small amount of sample into the system, provides the required carrier gas flow for the column, and may also perform other functions, including the evaporation and concentration of the sample. Gas from the injector flows through the column from the inlet, injector end, and the individual components of the mixture emerge separately from the outlet end of the column at a time that depends on the velocity of the carrier gas and the extent to which the components are slowed by the stationary phase.
These separated compounds are then passed into a device called a detector, which can simultaneously detect the presence of the components and, in general, measure the amount of each one present. The detector produces an electrical signal that is amplified and passed to data-processing equipment which measures both the time after injection that the component emerged from the column and also the amount of signal produced; it can then produce a report on the composition of the original mixture, which can be utilised by the User to determine what actions, if any, should be taken.
There are several points in this analytical process at which errors can be introduced. Two of these are at and before the point of injection.
Sample mixtures may be derived from a number of sources, and may require a variety of preparations, including concentration, extraction and reaction. In order to maintain the integrity of the sample, these steps are kept to a minimum and, wherever possible, they are automated. It is also important that the skill level of those preparing the samples does not affect the integrity of the results.
Samples may also be dirty. That is, they may contain materials that remain in the injector, may not pass through the column at all, or may contaminate the detector. Such materials can cause the deterioration of the results of subsequent analyses.
To overcome some of these problems, injection liners are frequently used in the injector of the chromatography An injection liner is a narrow tube that fits inside the injector interior, literally providing a liner for the injector, and samples are injected into this liner tube rather than directly into the injector itself. Materials that would remain in the injector are thus deposited and remain in the liner instead, and when the liner becomes too contaminated with these materials it can be removed and replaced with a new liner.
Injection liners may also deliberately contain materials that selectively hold back compounds, by chemical or physical processes. These materials are called packing materials, and are able selectively to absorb specific components of the mixture.
In some instances the absorption of components in the liner packing material is reversible. That is, a material may be absorbed at a low temperature, and may thereafter be released by raising the temperature. One such packing material absorbs organic components at a low temperature, but allows water to pass freely through it at the same low temperature; the water can thus be diverted out of the system whilst the organic materials remain within the packing material. Subsequently, the gas flows within the system can be redirected, and the liner and its contents heated up so that the organic materials are then displaced into the column for analysis.
However, the liners themselves become contaminated, and need to be changed from time to time. This is frequently a difficult process, and one that is also very difficult to automate. It involves first cooling the injector, and then reducing the gas pressure in the chromatography system. When the pressure inside the injector is at atmospheric pressure, the top of the injector is opened. The top of the injector generally has a number of pipes attached to it. Getting the top off with the pipes attached is an operation requiring considerable operator skill and dexterity. Once the top is out of the way, the liner can be pulled out and removed. A new liner must then be placed in the injector, and the above process repeated in reverse before the next analysis can be started.
This replacement process involves significant skills and a number of steps at which leaks and other errors can be introduced. The process is also very difficult to automate. Even in an otherwise automatic system, the replacement of the liner tends to be a manual operation. This means, unfortunately, that very often the liner is left in place far longer than it should be, leading to deterioration of the performance of the injector, possible degradation of samples in the injector, and potential errors in the results presented to the analyst.
The present invention proposes a novel type of injection liner that can more easily be replaced either manually or automatically. More specifically, the invention suggests an injection liner the input end of which has a laterally-extending external flange against which in use there can be placed an injection septum for closing and sealing the input end, which septum is then sealed in place against the flange by a closure cap that fits tightly over and around the two. For use with an injector that itself directly supplies the carrier gas, the liner carries sealing means beyond the flange which can in use form a seal against the interior surface of the injector, so separating this region from the atmosphere and also from the injector""s interior pressure.
In one aspect, therefore, the invention provides an injection liner for use in the injector of a gas chromatograph, the liner being in the form of an elongate tube having an input end and an output end,
the input end bearing a laterally-extending external flange against which in use there can be sealed an injection septum closing the input end, a closure cap then fitting tightly over and around the septum and the flange so as to hold the septum in place.
The invention also provides the xe2x80x9cusexe2x80x9d combination of liner tube, septum and cap. Thus, it provides an injection liner in the form of an elongate tube having an input end and an output end, the input end bearing a laterally-extending external flange, and the liner including
an injection septum, for fitting over the input end and sealing against the flange, and
a closure cap for fitting tightly over and around the septum and the flange so as to hold the septum sealingly against the flange and close the input end.
The invention provides an injection liner for use in the injector of a gas chromatograph. The idea of gas chromatography, and of the injector used as part of the apparatus, and the utilisation of a liner for the injector, has been discussed hereinbefore, and needs no further comment at this time, save perhaps to note that the chromatograph may be one of those where the carrier gas flow has to be fed into the liner via a narrow tube passing through the septum or it may be one of those where the injector can be modified to supply the gas directly, and it passes into the liner through a special input aperture positioned between its ends and on the atmosphere side of the sealing means which seals the liner to the inside of the injector. This is discussed further hereinafter.
The invention""s injection liner is in the form of an elongate tube having an input end and an output end. Although the dimensions of liners are chosen to fit the injectors with which they are to be utilised, a typical such liner is 80 mm (3.2 in) long with an internal diameter of 3 mm (xe2x85x9 in) and an external diameter of 5 mm (0.2 in), and is made of a borosilicate glass such as PYREX (Registered Trademark).
The liner""s input end carries the flange against which the septum seals; the output endxe2x80x94or, rather, a short length of the tube leading to the output endxe2x80x94may conveniently be associated with a restriction either to improve the liner""s ability to retain packing materials which may be placed therein, or to improve its ability to induce gas flows within it which encourage desired flow patterns, or both. Such a restriction can. take many forms; for instance, the tube may be internally narrowedxe2x80x94profiledxe2x80x94to provide one or more, step or waisted portion, or the tube may be blocked with a porous plug.
The input end of the liner of the invention bears a laterally-extending flangexe2x80x94an outwardly-directed onexe2x80x94against which there can in use be sealed an injection septum closing the input end. The flange need not be very deep, provided it can support the septum pushed against it; a simple lip of three or four millimetre (an eighth of an inch or so) is usually sufficient. The flange is conveniently given a number of concentrically-circular ridges on its septum-facing surface so as in use to assist in maintaining a gas-tight seal between the septum and the flange.
In use, the liner""s flanged output end has sealingly positioned against it an injection septum closing the input end. The purpose of this septum, a disc-like object matching the dimensions of the liner""s flanged input end, is to close that input end while at the same time allowing a sample (of the material to be analysed) to be injected into the liner through the septum using a syringe-like object fitted with a fine needle that can be pushed through the septum and then withdrawn therefrom without leaving any significant hole. The septum must therefore be made of a rubbery material; a preferred such material is a suitable silicone rubber, and advantageously this is protected by a coating on each side in the form of a thin layer of poly-tetrafluoroethylene (PTFE).
In use, the septum closing the liner""s input end is held in place with a closure cap fitting tightly over and around the septum and the flange. The cap can be screwed on (the exterior edge of the flange can constitute a suitable thread), it can be a shrink-fit over the septum and flange, or it can be crimped on (that is, be held tightly in place by distorting the material of the cap below the flange).
The cap may be made from, or enclose, magnetic material to assist in the manual or automatic removal and replacement of the liner, when required.
Conveniently, the cap extends in use laterally beyond the boundaries of the tube and flange, and can be used to provide a sealing surface against the input end of the injector.
Wherexe2x80x94as noted hereinbeforexe2x80x94the liner is to be employed with a chromatograph which is one of those where the injector is able to supply the carrier gas directly, the liner includes a special gas input aperture positioned between its ends (and usually closely adjacent the input end), and between this aperture and the liner""s output end there is sealing means which in use seals the liner to the inside of the injector. The sealing means is preferably an O-ring, suitably mounted around the exterior of the liner within a groove, or between wall-like ridges effectively defining such a groove, in/on the outside of the liner.
When in place in the injector, the liner may be held sealed in place by a downward force on the liner. This force would maintain the seal between the gas pressure inside the injector and the pressure of the surrounding ambient atmosphere.
In an application of the invention, individual samples may be stored in individual liners in an automatic or manual system for later analysis. The samples are placed in the liner, either directly or on to packing material as normal. The injection septum and cap are then fitted to the liner, and the complete assembly is stored in a storage container. When the sample is to be analysed, the liner, together with its septum and cap, are transferred to the injection port of the gas chromatograph, and sealed in place.
Injections are made into the liner and, when required, the liner is easily removed by reducing the gas pressure in the injector, removing the sealing force and simply withdrawing the liner manually or automatically, for example, with a magnet.